In a production environment, it is important to determine when jobs should be released for processing. A job is a number of items to be processed, or a number of tasks to be completed. Processing of an item may for example comprise building or assembling a product, or a part of a product, from raw materials or components. The terms “items”, “products” and “parts” may be used interchangeably to refer to units to be processed. A job may be governed by a “work order” which describes the set of operations to be performed to process a specified number of units of a particular item (or group of items) or the set of operations to be performed to complete a particular task. In certain production environments (e.g. in the semiconductor field), jobs may be referred to as “lots”.
Jobs have various attributes, such as a desired quantity of units, item characteristics, release date, due date, and the like, which may vary from job to job. Although many jobs be associated with a single type of item (e.g. a particular product), some jobs may be associated with two or more different types of items having similar characteristics. For example, in the semiconductor industry, items may be grouped into jobs based on product attributes, such as package type, lead count, pad size, device type and the associated motherlot, waferlot, and sublot identifiers.
Jobs are processed by machines. Machines may be grouped into work centers. A “work center” is a set of one or more machines having similar capabilities, each machine being capable of performing the same general processing operation. For example, in a semiconductor backend assembly environment, a “Wire Bond” work center may contain multiple wire bonders which come from different equipment manufacturers, yet they are all generally capable of completing the wire bonding operation. The terms “work center” and “operation” are sometimes used interchangeably.
Job release is an important and critical step in production planning. In the field of production planning, “job release” refers to the release of jobs from a pool of pending jobs for processing by machines on the shop floor. Job release scheduling dictates which jobs are to be released to the shop floor, at what time, and in what quantity of units (i.e. whether the job will be fully released or partially released). A job is said to be fully or 100% released to the shop floor when processing of all of the units comprising the job has been designated to commence during a particular time interval. A job is said to be partially released when processing of only some of the units comprising the job has been designated to commence during a particular time interval. For clarity, a distinction should be made between the release of a job (i.e. designation of at least a portion of the job for processing during a time interval) and the actual commencement of processing: a job may be fully or partially released even though no processing of the job has yet commenced. When a job is partially released, it may be deemed to be “X % released”, where X is a percentage determined from the ratio of the number of units that have been released over the total number of desired units in the job.
Regardless of whether a job is fully released or partially released during a particular time interval, processing of the released quantity is expected to complete during that time interval. In the case of a partially released job, it is usually desirable to release the unreleased portion of the job for processing in an immediately following time interval or intervals. “Job dispatching” refers to the scheduling of released jobs already on the production floor. Released jobs could be yet to be processed, i.e. queued at the first work center; currently being processed at a work center; or currently queued at an intermediate work center. In conventional production planning, job release and job dispatching are planned separately. It is said that job release control plays a more significant role than job dispatching in effective shop floor scheduling. On one hand, it is inefficient to release too many jobs at a time because some jobs will not be completed in a scheduled period of time and materials have to be stored on site until job completion. On the other hand, if too few jobs or the wrong types of jobs are released, production capacity will not be utilized to its full potential.
Various job release strategies are known. Generally, the known strategies control the job release to maintain a certain production parameter at a constant level. For example, the CONWIP (constant WIP) strategy attempts to maintain a constant Work-in-Process (WIP) level in the production. As is known in the art, WIP generally refers to a product or products in various stages of completion throughout a processing facility, including raw material that has been released for initial processing through to completely processed products awaiting final inspection. The Bottleneck strategy attempts to maintain an appropriate work load at a bottleneck operation. A bottleneck operation is an operation having limited processing capacity which may restrict the number of units of an item that can be processed during a time interval. The Global input/output strategy attempts to maintain a zero differential between the number of jobs released and completed over a given time interval. The Fixed Quantity Release strategy attempts to achieve a desired production output target.
Each of these known strategies has certain drawbacks. While studies have shown that the CONWIP strategy performs significantly better than the Bottleneck strategy, and both perform better than the other known strategies in a wide variety of conditions, in wafer fabrication facilities, it has been observed that application of the CONWIP strategy or its variants, while possibly resulting in reduced variability of WIP and cycle time of the wafer fabrication facility, tend to increase WIP level and cycle time of the job pool. Further, determination of the appropriate WIP level is often a difficult task. If the WIP level is too high, the control over the production floor will decrease. If the WIP level is too low, resources or production capacity will not be sufficiently utilized.
Further, with known job release strategies, long setup time is often required between two consecutive planning periods. Setup time is the time required for a specific machine, resource, work center, or production line to convert from the production of the last good piece of an earlier released lot to the first good piece of a later released lot.
Thus, there is a need for an improved job release strategy.